Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or so ordered as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size; the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties which are highly directional. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superimposed layers or laminae of carbon atoms joined together by weak van der Waals forces between layer planes, but strong covalent bonds exist within layer planes. In considering the graphite structure, two axes or directions are usually noted, to wit, the c axis or direction and the a axis or direction. For simplicity, the c axis or direction may be considered as the direction perpendicular to the carbon layers. The a axis or direction may be considered as the directions parallel to the carbon layers or the direction perpendicular to the c direction.
Among the graphites which may exhibit or possess a high degree of orientation, mention may be made of natural graphites, Kish graphite and synthetic graphites such as for example, pyrolytic graphites. Natural graphites are generally found or obtained in the form of small, soft flakes or powder. Kish graphite is the excess carbon which crystallizes out in the course of smelting iron. The graphite separates as fine flakes and is very similar to flake natural graphite. Synthetic graphites are produced by the pyrolysis or thermal decomposition of a carbonaceous gas on a suitable substrate or mandrel heated at an elevated temperature. The graphite, usually in the form of a massive, coherent deposit can be separated from the substrate in the form of coherent masses or bodies. If desired, the graphite masses can be pulverized, comminuted, shaved, or the like to produce synthetic graphite particles, e.g. powder, chip, flake, or the like of any desired size.
As noted above, the bending forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. It has been found that graphites having a high degree of orientation such as, for example, natural graphites, Kish graphite and synthetic graphites and heat treated pyrolytic graphites can be treated so that the spacing between the superimposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the c direction and thus form an expanded or intumesced graphite structure in which the laminar character is substantially retained.
In U.S. Pat. Nos. 1,137,373 and 1,191,383, natural graphite in the form of flake or powder of a size too great to pass through a 200 mesh screen is expanded by first subjecting the graphite particles for a suitable period of time to a oxidizing environment or medium maintained at a suitable temperature. Upon completion of the oxidizing treatment, the soggy graphite particles or masses are washed with water and then heated to between about 350.degree. C. and 600.degree. C. to more fully expand the graphite particles in the c direction. The oxidizing mediums disclosed are mixtures of sulfuric and nitric acids and mixtures of nitric acid and potassium chlorate.
By the above treatment, expansions of the natural graphite particles of up to about 25 times the original bulk were obtained. There is also disclosed that the expanded natural graphite can be compounded with a binder, e.g. a phenolic resin and the resultant composition compressed or molded into various forms, such as discs, rings, rods, foil, sheets, etc.
The sheet material formed from graphite particles having the desired degree of expansion also possesses substantial flexibility or pliability and can be made to have a density within the range of from about 5 pounds per cubic foot to a density approaching theoretical, that is, about 147 pounds per cubic foot.
In addition to the unique advantage of flexibility, the sheet material has also been found to possess an appreciable degree of anisotropy. Sheet material can be produced which has excellent flexibility, good strength and a high degree of orientation. Such highly oriented material possesses the excellent anisotropy or highly directional properties of pyrolytic graphite.
Briefly, the process of producing flexible, binderless graphite sheet material, e.g. web, paper, strip, tape, foil, mat or the like comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a c direction dimension which is at least 80 times, and preferably 200 times, that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet or foil. It should be noted that the expanded graphite particles which generally are worm-like or vermiform in appearance once compressed will maintain the compression set. The density and thickness of the sheet material can be varied by controlling the degree of compression.
Additionally, other methods for inducing, expanding and compressing condition to the graphite may be employed. These methods include chemical treatment of natural graphite and heat treated pyrolytic graphite particles are treated over predetermined time periods to a variety of oxidizing or intercalating solutions (e.g. H.sub.2 SO.sub.4, HNO.sub.3, KMnO.sub.4, FeCl.sub.3, etc.) at temperatures ranging from about room temperature to about 125.degree. C. The graphite particles utilized can range in size from a dust or fine powder small enough to pass through a 325 mesh screen to a size such that no dimension is greater than about one inch.
Additionally, various oxidizing agents and oxidizing mixtures may be employed to effect a controlled interlayer attack of the graphite particles. Such agents are, for example, nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid and the like. The mixtures used to achieve interlayer attack of the graphites may be, for example concentrated nitric acid and potassium chlorate, chromic acid and phosphoric acid, sulphuric acid and nitric acid etc. or mixtures of a strong organic acid, (e.g. trifluoroacetic acid ) in a strong oxidizing agent soluble in the organic acid used. As will be appreciated by those skilled in the art, a wide range of oxidizing agent concentrations can be utilized.
It will also be appreciated that the opening up or spreading apart of carbon layers can be achieved by chemically treating the graphite particles with an intercalating solution or medium so as to insert or intercalate a suitable additive between the carbon hexagon networks and thus form an addition or intercalation compound of graphite. For a more detailed discussion of processes for forming and treating graphite in order to promote interlayer attack reference should be made to U.S. Pat. No. 3,404,061 issued to J. H. Shane, et al.
Once the desired expanded graphite is obtained, the expanded graphite is typically fed into an apparatus comprising a conveyor and various rollers for pressing and forming the graphite into a continuous sheet of foil.
However, a problem inherent in all prior art methods of forming continuous sheets of graphite foil is that when the graphite foil is placed in a high temperature and/or low pressure environment, in which the material is typically used, the surface texture of the graphite deforms such that bubble-like deformations appear on the graphite foil. The formations are believed to be due to reexpansion of the expanded graphite, release of gases, moisture or other factors. The bubble-like deformations detract from the cosmetic appearance of the foil and may, under mechanical or other pressure, eventually burst and possible release the expanded graphite flakes into the surrounding environment.
Since graphite sheets are commonly used as insulators and heat shields, and thus, many uses of graphite foil involve the application in high temperature and/or low pressure environments (i.e. as when the foil is used to line vacuum or standard furnaces as insulation) the deformation of the graphite foil is a particularly common, troublesome and undesirable occurrence. One prior art method attempting to solve this problem is disclosed in U.S. Pat. No. 3,404,061 to Shane et al. In Shane et al., after the graphite material is rolled into a sheet, the foil then passes through a heating means which heats the graphite to an elevated temperature, e.g. 1000.degree. C., so as to cause re-expansion of compressed particles which were not previously completely expanded or which contain residual fluid, gases, etc. After the foil is heated, the graphite sheet is then passed through a final rolling process to recompress the foil to a desired thickness. Of course, the application of high temperature to the already processed foil is time consuming, expensive and is not completely effective in alleviating the above-described problem. Another potential way to avoid the deformation problem is to utilize only highly purified salts in the manufacture of the graphite foil. However, such materials are more expensive and less available than the materials typically used in the commerical manufacture of graphite foil material and would accordingly raise the cost of the graphite foil material and slow down the manufacturing process. It is also very difficult to ensure the purity of the salts.